This invention relates to an electrophotographic copying apparatus or more particularly, to an electrophotographic image density control or a photoreceptor charge control arrangement.
In the situation of varying temperature and humidity characteristics, and further, of consecutive operating duties of an electrophotographic copying apparatus, the steady and uniform distribution of the surface potential on the photoreceptor is not generally expected, causing an undesirable influences on the reproducibility of the images to be transferred, or especially, on the reliable reproducibility of the electrophotographic image density.
More specifically, in the copying apparatus of latent image transfer type wherein a latent image created on the electrophotographic photoreceptor is electrostatically transferred onto a copy paper composed of respective insulating and conductive layers and the image mentioned above is successively developed, the non-uniform distribution or, variation of the surface potential on the above-mentioned photoreceptor is quite undesirable, due to the fact that the variation of the surface potential on the above-mentioned photoreceptor substantially affects the reproducibility of the image to be copied. That is to say, the surface potential relatively representing the original portion having a low picture image density on the surface of the photoreceptor is to be approximately the same level as that to be rendered by a white portion of the original, i.e., non-image bearing portion of the original. Therefore, if the surface potential is undesirably varied from a fixed reference due to the reasons mentioned above in the course of the latent image transferring steps, the transfer of the original portion having a low image density mentioned above is not naturally performed satisfactorily, thus resulting in dimming and further, even fogging of the copied images in the case where the white portion mentioned above is undesirably charged. Such undesirable situations as described in the foregoing is inherent in the electrostatic copying process and is concerned with the copying process even with the copying apparatus of powder image transferring type, in which the predetermined relation between the surface potential of the photoreceptor and the biasing force to be specifically employed for development is to be resultantly varied.
Accordingly, in order to overcoming the inherent disadvantages mentioned above and thereby, to constantly obtain copies having a predetermined image density, various methods comprising a step for detecting the surface potential of the charged photoreceptor and a successive step for controlling the charge amount to be charged onto the photoreceptor subject to the above-mentioned detection have already been proposed.
For example, according to the invention disclosed in U.S. Pat. No. 3,788,739, the photoreceptor has a portion for detection within the area exposed to an image projection so as to be simultaneously exposed to the maximum exposure radiation intensity. Accordingly, measurement of the surface potential of the portion for detection, which has already been exposed as mentioned above, and successive comparison of the surface potential to the fixed reference are arranged to provide a control instruction which can be employed to compensate charge, exposure, transfer and development. However, in the known arrangement as described above, since an electrometer or surface potential detector is disposed in a position following the charging and exposing stations arranged along a process route, even if the arrangement may be capable of controlling charge, exposure, and development in a manner as mentioned above and further maintaining the surface potential to be apparently constant in view of an integral time scale of run, apriori compensation of charge etc. can hardly be accomplished.
More specifically, as may be clear from the discription in the foregoing, the surface potential correspondingly prevailing between the charger and the electrometer can not be compensated, even if the electrometer is to detect the variation of the surface potential on the photoreceptor, which may be caused by the electrical parameters specifically concerning the charger and the radiation lump and/or the successive changes of the temperature and humidity as described earlier. Moreover, such being the case as mentioned above, if the surface potential is to be kept steady in respect to the fixed reference, with the charger and radiation source etc. being momentarily controlled, through the momentary signal output from the electrometer, an oscillation phenomenon, which is to be effected in a control circuitry due to the existence of the specific control time lag, can not be avoided. Therefore, according to the arrangement mentioned above, the momentary outputs are arranged to be impressed onto the control circuitry with the help of an integrating circuit means. However, even such the case as mentioned above, if the capacity of a capacitor to be employed in the integrating circuit means is not large enough, it is natural to take a long period of time before the oscillation is stabilized, or the control mentioned above is often rendered to be unstable. On the contrary, if the capacity of the capacitor is excessive for the intended purpose, the response time for the respective compensation tends to be delayed. Furthermore, even if the capacity of the capacitor is predetermined so as to be in an appropriate range within the two extreme conditions as described in the foregoing, the possible capacity of the control circuit for controlling the respective extremes mentioned above are diminished, thus resulting in a state in which the control circuit means mentioned above is not adaptable for compensating the rather large variation of the surface potential.
Moreover, if the photoreceptor is made of a cylindrical drum and has a deposited or spattering layer of photoconductive material thereon then, the non-uniform deposition of photoconductive material on the drum is naturally expected and therefore, becomes one of the reasons causing the surface potential to be undesirably distributed. Consequently, such being the case as mentioned above, other difficulties for accomplishing an appropriate control are further encountered, as long as the separate positioning of the electrometer with respect to the control circuit means including the charger or the exposing station etc. is left unchanged, the situation of which can be found in the prior art as described in the foregoing.